1. Field of the Invention
This invention relates to circuits for use with an electric motor supplied from a DC voltage source, and, in particular, to a circuit comprising a commutating arrangement having transistors which are provided by a control circuit with on-off commands in dependence on the motor rotor position.
2. Background of the Invention
A circuit of the above type is disclosed in the journal "Technische Rundschau", no. 2, Jan. 15, 1974, page 27. In the commutating arrangement of the circuit, commutation is accomplished by appropriately controlling the on-off states of the transistors in the two bridge halves. During each commutation, a transistor in one bridge half of a bridge branch is controlled to be off, while the transistor situated in the other bridge half of the same bridge branch is controlled to be on. Because of the finite switching times of the transistors, the transistors of a bridge branch are both on for a short duration of time during the commutation process. During this overlap of the switching times of the transistors the DC voltage source is short-circuited and the current in the circuit is thus limited only by the internal resistance of the source. This presents a considerable load for the source and also for the transistors which for a time must operate in the region of second breakdown. As a result, the life of the transistors is considerably shortened. Moreover, to provide interlocking between the transistors arranged in the same bridge branch to alleviate the above condition is too expensive as, in part, fixed potentials are not present.
It is, therefore an object of the present invention to provide a circuit including a commutating arrangement for an electric motor supplied from a DC voltage source which is designated to inhibit excessive loading of the DC voltage source and the transistors of the commutating arrangements.